This application claims the priority of Korean Patent Application No. 2003-73442 filed on Oct. 21, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a light-emitting device, and more particularly, to a high efficiency light-emitting device with improved light extraction efficiency and good defect density control and stress distribution control and, in which, a substrate limits a surface crystal orientation.
2. Description of the Related Art
In general, light-emitting devices include laser diodes (LD) and light emitting diodes (LED), and LEDs use properties of compound semiconductors to transmit a signal, which is electric energy converted into an infra-red light, visible light, or other forms of light. The converting of electric energy into light can be categorized into temperature radiation and luminescence. Photo luminescence caused by the excitation of light, a cathode luminescence caused by the irradiation of x-ray or an electron beam, and electroluminescence (EL) are all types of luminescence. An LED is one kind of an EL and currently LEDs using group Ill-V compound semiconductors are widely used.
Group III nitride compound semiconductors are direct transition semiconductors, and are widely used in light-emitting devices such as LEDs and LDs since it is possible to obtain stable operation at a high temperature than devices that use other semiconductors. In general, the Group III nitride compound semiconductors use sapphire (Al2O3) as a substrate and are formed on top of the substrate.
FIG. 1 is a cross-sectional view of in general Group III nitride compound semiconductor, including a sapphire substrate. A n-GaN layer 12, an active layer 13, a p-GaN layer 14, and a p-type electrode layer 15 are sequentially formed on a sapphire substrate 11. In addition, an n-type electrode layer 16 is formed on the n-GaN layer 12 where the active layer 13 is not formed. In a general LED, the most important issue is how efficiently can the light, which is created at an internal active layer, be extracted externally.
To efficiently extract light created in the longitudinal direction of the sapphire substrate and active layer, efforts to form transparent electrodes or reflective layers have been made. However, a large amount of light, which is created at the active layer, is transmitted in a latitudinal direction. Therefore to extract the light in a longitudinal direction, various methods such as forming the side walls of the structure of accumulative layers of a semiconductor device to have a predetermined angle, and forming side walls composed of reflective material have been made, but this caused problems in the manufacturing process and increased costs. Furthermore, to increase the light emitting ability of Group III nitride compound semiconductor light-emitting devices that use a sapphire substrate, a device with a flip chip-type is adopted and the light extraction efficiency is at approximately 40% due to the difference in diffraction rates between the GaN and sapphire substrate.
Recently, as shown in FIG. 2a, an LED structure in which an uneven structure is formed by processing the surface of a sapphire substrate 21 and forming semiconductor crystal layers, which include active layers, on top of the substrate has been introduced. Such a structure forms a diffraction rate interface having an uneven surface under the active layer 22 and enables the external extraction of a portion of the light which fades out within the device.
In addition, when forming a Group III nitride compound semiconductor on the sapphire substrate 21, a dislocation occurs due to the miss fit of the sapphire substrate 21 and the lattice parameters of a Group III nitride compound semiconductor. To prevent this, as shown in FIG. 2b, the sapphire substrate 21 has an uneven surface and a GaN layer 23 is formed on top. FIG. 2c schematically illustrates a process of forming an LED on top of a sapphire substrate which has such an uneven structure. When forming the GaN layer 23 on top of the sapphire substrate 21 which has an uneven structure as shown in FIGS. 2c-(a), GaN facets are grown 24 from the top and each side portion of the uneven structure, as shown in and 2c-(b). Then a planarized GaN layer 23 can be obtained as shown in FIG. 2c-(c). FIG. 2c-(d) illustrates the completion of a light emitting diode, in which an active layer 22, etc. are on top of the planarized GaN layer 23.
This process has a disadvantage in that when growing the semiconductor crystal layer using such a patterned sapphire substrate (PSS), since planarization is carried out after facet growth is performed on the pattern, regrowth has to be done to a sufficient thickness to perform planarization.
In addition, a structure is disclosed (No. WO2001-69663), in which a step difference is formed, group III nitride compound semiconductors are grown on the top surface and side portions of the step difference and a piercing phase is prevented. However, a disadvantage is that a void is formed in the lower portion of the step difference and to planarize the growth layer group III nitride compound semiconductors have to be formed relatively thick.
When regrowing the semiconductor on the sapphire substrate, an ELOG and a PENDEO method are used to reduce the defect density. However, in the case of the ELOG method a separate mask layer is needed, and in the case of the PENDEO method, a void is formed on the interface portion of the substrate resulting in a decrease in light extraction efficiency.